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<title>Adding new DECL node types (GNU Compiler Collection (GCC) Internals)</title>

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<h4 class="subsubsection" id="Adding-new-DECL-node-types-1"><span>11.4.2.2 Adding new DECL node types<a class="copiable-link" href="#Adding-new-DECL-node-types-1"> &para;</a></span></h4>

<p>Adding a new <code class="code">DECL</code> tree consists of the following steps
</p>
<dl class="table">
<dt>Add a new tree code for the <code class="code">DECL</code> node</dt>
<dd><p>For language specific <code class="code">DECL</code> nodes, there is a <samp class="file">.def</samp> file
in each frontend directory where the tree code should be added.
For <code class="code">DECL</code> nodes that are part of the middle-end, the code should
be added to <samp class="file">tree.def</samp>.
</p>
</dd>
<dt>Create a new structure type for the <code class="code">DECL</code> node</dt>
<dd><p>These structures should inherit from one of the existing structures in
the language hierarchy by using that structure as the first member.
</p>
<div class="example smallexample">
<pre class="example-preformatted">struct tree_foo_decl
{
   struct tree_decl_with_vis common;
}
</pre></div>

<p>Would create a structure name <code class="code">tree_foo_decl</code> that inherits from
<code class="code">struct tree_decl_with_vis</code>.
</p>
<p>For language specific <code class="code">DECL</code> nodes, this new structure type
should go in the appropriate <samp class="file">.h</samp> file.
For <code class="code">DECL</code> nodes that are part of the middle-end, the structure
type should go in <samp class="file">tree.h</samp>.
</p>
</dd>
<dt>Add a member to the tree structure enumerator for the node</dt>
<dd><p>For garbage collection and dynamic checking purposes, each <code class="code">DECL</code>
node structure type is required to have a unique enumerator value
specified with it.
For language specific <code class="code">DECL</code> nodes, this new enumerator value
should go in the appropriate <samp class="file">.def</samp> file.
For <code class="code">DECL</code> nodes that are part of the middle-end, the enumerator
values are specified in <samp class="file">treestruct.def</samp>.
</p>
</dd>
<dt>Update <code class="code">union tree_node</code></dt>
<dd><p>In order to make your new structure type usable, it must be added to
<code class="code">union tree_node</code>.
For language specific <code class="code">DECL</code> nodes, a new entry should be added
to the appropriate <samp class="file">.h</samp> file of the form
</p><div class="example smallexample">
<pre class="example-preformatted">  struct tree_foo_decl GTY ((tag (&quot;TS_VAR_DECL&quot;))) foo_decl;
</pre></div>
<p>For <code class="code">DECL</code> nodes that are part of the middle-end, the additional
member goes directly into <code class="code">union tree_node</code> in <samp class="file">tree.h</samp>.
</p>
</dd>
<dt>Update dynamic checking info</dt>
<dd><p>In order to be able to check whether accessing a named portion of
<code class="code">union tree_node</code> is legal, and whether a certain <code class="code">DECL</code> node
contains one of the enumerated <code class="code">DECL</code> node structures in the
hierarchy, a simple lookup table is used.
This lookup table needs to be kept up to date with the tree structure
hierarchy, or else checking and containment macros will fail
inappropriately.
</p>
<p>For language specific <code class="code">DECL</code> nodes, there is an <code class="code">init_ts</code>
function in an appropriate <samp class="file">.c</samp> file, which initializes the lookup
table.
Code setting up the table for new <code class="code">DECL</code> nodes should be added
there.
For each <code class="code">DECL</code> tree code and enumerator value representing a
member of the inheritance  hierarchy, the table should contain 1 if
that tree code inherits (directly or indirectly) from that member.
Thus, a <code class="code">FOO_DECL</code> node derived from <code class="code">struct decl_with_rtl</code>,
and enumerator value <code class="code">TS_FOO_DECL</code>, would be set up as follows
</p><div class="example smallexample">
<pre class="example-preformatted">tree_contains_struct[FOO_DECL][TS_FOO_DECL] = 1;
tree_contains_struct[FOO_DECL][TS_DECL_WRTL] = 1;
tree_contains_struct[FOO_DECL][TS_DECL_COMMON] = 1;
tree_contains_struct[FOO_DECL][TS_DECL_MINIMAL] = 1;
</pre></div>

<p>For <code class="code">DECL</code> nodes that are part of the middle-end, the setup code
goes into <samp class="file">tree.cc</samp>.
</p>
</dd>
<dt>Add macros to access any new fields and flags</dt>
<dd>
<p>Each added field or flag should have a macro that is used to access
it, that performs appropriate checking to ensure only the right type of
<code class="code">DECL</code> nodes access the field.
</p>
<p>These macros generally take the following form
</p><div class="example smallexample">
<pre class="example-preformatted">#define FOO_DECL_FIELDNAME(NODE) FOO_DECL_CHECK(NODE)-&gt;foo_decl.fieldname
</pre></div>
<p>However, if the structure is simply a base class for further
structures, something like the following should be used
</p><div class="example smallexample">
<pre class="example-preformatted">#define BASE_STRUCT_CHECK(T) CONTAINS_STRUCT_CHECK(T, TS_BASE_STRUCT)
#define BASE_STRUCT_FIELDNAME(NODE) \
   (BASE_STRUCT_CHECK(NODE)-&gt;base_struct.fieldname
</pre></div>

<p>Reading them from the generated <samp class="file">all-tree.def</samp> file (which in
turn includes all the <samp class="file">tree.def</samp> files), <samp class="file">gencheck.cc</samp> is
used during GCC&rsquo;s build to generate the <code class="code">*_CHECK</code> macros for all
tree codes.
</p>
</dd>
</dl>


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